When the company exited reorganization in 1916, it was renamed to Chicago North Shore and Milwaukee Railroad. Chicago utilitymagnateSamuel Insull acquired a controlling interest in the railroad and served as its chairman. Insull, through a holding company, controlled two other Chicago-area interurban railroads — the Chicago Aurora and Elgin Railroad which connected Chicago with its west suburbs and the Fox River Valley, and the Chicago South Shore and South Bend Railroad or South Shore Line, which connected Chicago with northern Indiana and which continues to this day as one of the last interurbans in the United States.

Hat pin from motorman on the Chicago, North Shore and Milwaukee railroad.

The original arrangement at the railroad's southern terminus in Evanston was for Chicago-bound passengers to transfer to the Northwestern Elevated (one "L" company) there. Following the railroad's acquisition by Samuel Insull, limited-stop North Shore Line trains began connecting with special "L" trains (also controlled by Insull) that ran express to the Chicago Loop. In 1919, the North Shore Line obtained trackage rights over the Northwestern Elevated and modified their cars with third railequipment, giving them direct access to the Loop. The following year, a new terminal in Milwaukee was dedicated, and in succeeding years, the remaining single-track in Wisconsin was eliminated, with the exception of a one-half mile stretch of single-track in southern Milwaukee that remained a minor bottleneck until the railroad's end.

During the early 1920s, the railroad instituted a number of named, limited-stop trains, some carrying deluxe dining and parlor/observation cars. One of the railroad's most distinctive named trains, inaugurated in 1917, was the Gold Coast Limited. The North Shore also created a network of motor coach (bus) lines to feed on potential traffic from territory not directly served by the company's trains.

The growth of the north shore communities provided good traffic levels for the railroad, but the increasing congestion of these communities' business districts impeded the railroad's desire to remain competitive with the competing steam railroads for longer-haul passenger business, in particular the Chicago-Milwaukee traffic. The North Shore therefore sought to build a new bypass line through the Skokie Valley — what was then undeveloped rural land approximately four to five miles west of the lake shore route.

The needed real estate purchases and financing were arranged in 1923 and 1924, and construction of the new line began in April 1924. The new line diverged from the Howard Street "L" station located at the boundary between Chicago and Evanston, ran west into the village of Niles Center (now Skokie), continuing to the north-northwest from that point through marshy countryside, paralleling the Skokie branch of the Chicago and North Western Railway. At South Upton, the new route ran eastward along the North Shore's Mundelein branch until just west of Lake Bluff, at which point a new connection diverged to the north onto what had been a freight-only branch which connected to the original main line at North Chicago Junction.

An arrangement was made with the Chicago Rapid Transit Company, wherein local "L" service was begun over the new line to the Dempster Street station in Niles Center in 1925. It had been anticipated that the opening of the new "L" line would help launch a real estate boom in the area as it had decades earlier in other parts of the Chicago area. The Great Depression put a damper on the area's growth, and Niles Center (by that time renamed Skokie) didn't really begin to experience a surge of growth until the 1950s.

Though the Niles Center elevated service failed to prosper, the transit operator benefited from the construction of new shop facilities on vacant land along the southern part of the Skokie Valley line. This spacious facility relieved older, more crowded facilities on the "L" system and remains to this day as the Chicago Transit Authority's primary maintenance facility for its rail system.

The remaining portion of the North Shore Line's new Skokie Valley line entered service in 1926. The new route consisted of 18 miles (29 km) of new double-track railroad, and the route was a mere 2.5 miles (4 km) longer than the old main line. Because it traversed mostly rural area, higher speeds could be sustained for a longer distance. In conjunction with the completion of the Skokie Valley route, the railroad had improved the Mundelein branch, building a new terminal and double-tracking the branch. Mundelein had previously been served by shuttle service connecting with main line trains at Lake Bluff; with the opening of the new Skokie Valley line on June 5, 1926, North Shore inaugurated an hourly Chicago-Mundelein local suburban service, interspersed with the hourly Chicago-Milwaukee limited-stop trains. Diversion of the Chicago-Milwaukee service onto the Skokie Valley line brought a reduction in travel time of 20 minutes.

The original main line — now designated by the railroad as the Shore Line — continued to host Chicago-Waukegan service, which consisted of limited-stop Chicago-Waukegan service as well as all-stop local service, each operating at roughly 30 minute headways.

Initially after the stock market crash in 1929, business went on as usual, but as the depression deepened and as the Insull public utility empire began to crumble, the railroad entered receivership in 1932. The dire economic conditions and high unemployment caused ridership (and hence revenue) to plummet. A labor strike in 1938 precipitated by a 15% reduction in wages kept the railroad from operating for seven weeks.

In spite of the difficult conditions during the 1930s, the North Shore was able to undertake a major grade separation project along the Shore Line. The North Shore had for nearly a quarter century sought to eliminate the hazards and operating costs associated with running a busy railroad through the business districts of one built-up suburb after another. Prior to the Depression, grade separation projects had been funded by the railroads' private capital, and neither the North Shore Line nor the paralleling steam-operated Chicago and North Western Railway were in a financial position to undertake such a venture even before the stock market crash in 1929.

However, in 1937 President Franklin D. Roosevelt and his Secretary of the Interior, Harold Ickes (a Winnetka resident), announced a Public Works Administration program to "prime the pump" of the American economy. This timely program allowed the railroads and the communities of Winnetka and Glencoe to obtain federal funding for the grade separation of the two railroads through their business districts. The project was complicated by the need for construction work to take place under traffic — the two railroads combined operated more than 200 daily trains. The grade separation was completed in late 1941 — just nine weeks before the United States went to war — and cost $4.3 million.

To meet the competition of modern streamlined trains operating on the steam railroads connecting Chicago and Milwaukee, the North Shore in 1939 embarked on a program to modernize a portion of its steel coach fleet for both commuter and intercity service. Some 15 coaches dating from 1928 were modernized, practically from the ground up. All-electric heating was installed with a new ventilation system, new flooring, new interior decorations and fittings. The cars' exteriors were painted green with gray and red trim, and were dubbed "Greenliners". These cars were regularly assigned to Skokie Valley limited-stop service.

North Shore public timetable dated February 9, 1941, introducing the Electroliners

The most significant component of the passenger equipment modernization program was the purchase of two articulated streamlined trainsets. The trainsets consisted of four cars semi-permanently attached. The two end units included operating cabs and smoking and non-smoking coach seating. An additional car provided more coach seating, and the tavern/lounge car rounded out the four-car consist. These trains were dubbed Electroliners and were the railroad's premiere service run as fast as 90 mph between Dempster Street and North Chicago Junction on the North Shore's excellent track and roadbed. Entering service on February 9, 1941, each trainset was scheduled to run five one-way trips in Chicago-Milwaukee service every day. The Electroliners continued in service until the end of the railroad in 1963. The Electoliners were sold to the Philadelphia Suburban Transportation Company, and were renamed Liberty Liners. The Liberty Liners were retired around 1979.

The outbreak of World War II caused the railroads of the United States to see a sharp rise in traffic. Even before the imposition of rationing of fuel and rubber made auto travel difficult, the North Shore saw its freight and passenger traffic rise to record levels, in part due to the railroad serving important military facilities: the Army's Fort Sheridan just north of Highwood, and the Navy's Great Lakes Naval Training Station, just south of North Chicago. North Shore saw its traffic increase to the extent that the railroad was forced to borrow equipment from the Chicago Rapid Transit Company and fellow interurban Chicago Aurora and Elgin Railroad, both former Insull properties.

Wartime earnings were high enough that the railroad's bankruptcy trustees were able to pay some of the company's outstanding debt and submit a reorganization plan. After the plan was approved, a new corporation (with a different corporate name from before) assumed the property in 1946.

The failure to resolve a wage dispute taken to the National Mediation Board in 1948 led to a 91-day work stoppage that spring. The dispute was resolved by increasing both fares and wages, though the company's employees continued to earn less than their counterparts at other area railroads. Simultaneously, a decline in rail travel began as initial postwar shortages of automobiles ended. These national trends—coupled with the lost revenue from the three-month strike and the effects of the strike-settling wage increase—created serious passenger revenue losses for the line. In 1949, the railroad sought to curtail some of its more unprofitable services. Dining car service (other than that on the Electroliners) was dropped, service (particularly on the Shore Line) was reduced, and the railroad applied unsuccessfully to drop Shore Line service altogether.

When the franchise held by the North Shore subsidiary operating streetcar service in Waukegan expired in 1947, the company felt that a renewal was not justified, the company replaced its city operations there with bus service. Shore Line trains that used the streetcar tracks to reach downtown Waukegan were simultaneously cut back to allow the tracks to be abandoned. The subsidiary city streetcar service in Milwaukee was discontinued in 1951; trackage remained, however, as it was used by main line services to access the North Shore's Milwaukee terminal.

The railroad repeated its petition to abandon Shore Line in 1954. Though rush hour traffic levels remained strong, off-peak ridership had declined sharply, leading to further losses. The remaining street running and numerous stops eliminated many of the advantages of rail transportation on this route. Travel time on the Shore Line was roughly twice that of the slightly longer Skokie Valley route. The completion of the Edens Expressway through the Skokie Valley in late 1951 caused mounting ridership losses reflected on the railroad's earnings statements. Though the abandonment proceedings garnered strong opposition in the communities affected, the railroad was successful in proving its case and was authorized to end service on the Shore Line. July 24, 1955 was the final day of service on that route. A short portion of the line was retained to provide access from North Chicago Junction to the railroad's shops in Highwood; the remainder of the line was removed, much of the right-of-way becoming automobile parking spaces for commuters who switched to the suburban trains of the parallel Chicago and North Western Railway.

With its transportation holdings proving increasingly unprofitable, Susquehanna moved to cut its losses; in 1958, the railroad filed with state and federal regulatory authorities for the authority to discontinue all service and abandon the entire property. The Interstate Commerce Commission (ICC) examiner handling the case recommended abandonment, but the Illinois regulators recommended the continued operation of the railroad. For the time being, ridership remained fairly stable, but the completion of the Northwest Expressway (now the Kennedy Expressway) in late 1960 provided a link between the Edens Expressway and the Chicago Loop. The North Shore Line's passenger traffic began to hemorrhage at the rate of 46,000 passengers per month.

The Chicago Transit Authority researched[2] the possibility of continuing truncated rail service between Waukegan and Howard Street in Chicago, with buses assuming operations between Lake Bluff and Mundelein. The report, released that October, revealed that passenger service had dropped to an average of 14,000 daily riders, and that the line was in dire need of modernization. The report recommended that the CTA only assume operations under the conditions that the acquisition of the railroad's property and modernization of the fleet could be achieved without cost to the agency, and an operational subsidy would be provided. In February, 1961, an updated study was released, revealing that patronage had become even lighter than it had been when the initial study was conducted. Ultimately, no action was taken as a result of the study, as none of the recommended conditions could be met.

That February, the railroad requested expedited action by the ICC on its abandonment petition, citing its mounting losses. On May 17, 1962, the request was approved under the condition that no buyer stepped forward within 35 days. Both the Illinois regulators and an association of commuters opposed the action, the association offering to buy the railroad at salvage value but ultimately failing to raise sufficient funds to buy the property. That November, the state of Illinois ruled in favor of the ICC, and prevented the commuters association from having the abandonment postponed any further. The last full day of service came on January 20, 1963, with the final trains reaching their destination in the early hours of the following morning. Sporadic freight movements continued into the next week, as the remaining cars on the line were collected from various points on the system.

Most of the rails were removed in the succeeding two years. The Chicago Transit Authority purchased the southernmost portion of the Skokie Valley line between Howard Street and Dempster Street, Skokie, and in early 1964 obtained federal funding for what turned out to be a successful mass transportation pilot project, dubbing the new non-stop service as the "Skokie Swift." That same year, the Skokie Valley Transportation Council was formed[3] by the towns of Glenview, Northbrook, Northfield and Skokie, with the goal of reviving rail service by funding an extension of the "Skokie Swift" further north. This was prevented, however, by the sale of the trackage between Dempster Street and Lake-Cook Road to the Chicago & North Western Ry. for use as a freight line;[4] the Union Pacific (which the North Western was merged into in 1995) continued to operate the line until 2001, and it was dismantled in 2004-05. CTA is studying possible extension of the Yellow Line along the North Shore right-of-way as far as Old Orchard Road, opposite the Old Orchard shopping center.

The former North Shore right of way from the Illinois border to Milwaukee was sold off piecemeal to numerous private interests. Many homes and businesses have been built in what was the right of way. To try to use the right of way again would be virtually impossible. In Illinois, extension to the Skokie Swift into the now-fully-developed territory in the Skokie Valley is discussed periodically. In other places, parts of the North Shore right of way have been turned into paved trails, as part of the rails to trails program.